As dimensions of integrated circuit features reduce, the negative effects of roughness of the features, and/or of components such as photo-resist and ancillary structures used to produce the features, become more pronounced since there is no corresponding reduction of roughness with dimension reduction. As a result of the increased problems, metrics that quantify roughness of specific sections of an integrated circuit have been developed; for example, line edge roughness (LER) measures the roughness of a linear edge. One metric for LER that is used in the art is a value of 3σ, where σ is the standard deviation of points of the linear edge from a straight line. Other metrics have also been developed.
For example, definitions of edge roughness magnitude and spatial frequency for a linear edge are given by Nelson et al. in an article entitled “Comparison of Metrology Methods for Quantifying the Line Edge Roughness of Patterned Features,” in Journal of Vacuum Science and Technology B17(6), pages 2488-2498 (1999), which is incorporated herein by reference.
U.S. Patent Application 2003/0190069, to Arkady et al., which is incorporated herein by reference, describes a method for measuring the line edge roughness of an object using a scanning electron microscope. An edge of the object is oriented so that it is non-parallel to a direction of scan and the object is scanned in a line-by-line method. Edge points determined by the scan are fitted to a straight line, and a metric measuring the perpendicular deviation of these points from the line is derived.
U.S. Patent Application 2003/0021463, to Yamaguchi et al., which is incorporated herein by reference, describes a spatial frequency measure for LER. The measure is derived by performing a Fourier transform of points generating a linear edge, and using the Fourier coefficients to generate the frequency measure.
Vandeweyer et al., in an article entitled “The measurement of Contact Edge Roughness in 193 nm patterning,” in Interface 2003, (September, 2003), which is incorporated herein by reference, describe a process for measuring roughness of contact holes. The process fits edge points of the contacts to a circle, and a metric for the contact edge roughness is defined as the standard variation of the points from the circle.
Notwithstanding the metrics described above, there is a need for a metric that better quantifies roughness of features of an integrated circuit.